This invention relates to carbon-silicon carbide brake preforms manufactured by carbonizing a blend of carbon (e.g., polyacrylonitrile) fibers and thermosetting pitch resin to provide an intermediate product having open porosity and subsequently filling the pores of the intermediate product with silicon by a melt infiltration process.
Brake discs for aircraft or automobiles require materials having high heat resistance and long wear. Asbestos has been used in these applications, due to its heat resistance properties. Asbestos-based friction components have limited applicability under severe use conditions because the polymeric resins used to bind the asbestos fibers together decompose at elevated temperatures. Carbon-carbon brake components have become increasingly more common, but their high cost and poor cold friction properties precludes their use in automotive applications.
Among the types of substrates used to make carbonxe2x80x94carbon parts are discontinuous carbon fiber moldings, nonwoven airlaid carbon substrates, woven carbon fiber substrates, and braided carbon fiber substrates. The substrates are typically stacked on top of each other to a desired thickness, and then the stacked substrates are needle-punched together to join or consolidate the substrates to each other by intermingling carbon fibers between the layers of substrates in the preform so produced. The preform is then typically batch carbonized to char the fiber of the substrate and thereby increase the carbon content of the preform. The carbonized preform may then be die cut to a desired shape. Subsequently, carbon atoms may be deposited on the carbon fibers of the preforms by means of a chemical vapor deposition (CVD) process, e.g. with methane. The preform may then be heat-treated to reorient the carbon atoms which optimizes thermomechanical properties, machined if necessary, and treated with an anti-oxidant to form the finished carbonxe2x80x94carbon part.
U.S. Pat. No. 6,248,269, like the present application, relates to C/SiC brake preforms. This patent teaches manufacturing brake linings of fiber-reinforced C/SiC materials by producing carbon fiber bodies having specific pore and/or capillary volumes, infiltrating the carbon fiber bodies by carbon and/or carbon precursors, and pyrolyzing the infiltrated carbon fiber body to produce porous C/C pre-bodies which are then infiltrated by liquid silicon, at which point xe2x80x9cthe carbon is ceramized to silicon carbide at least in the area of the pores and capillaries close to the surfacexe2x80x9d. Column 3, lines 11-24. As will be apparent from the description herein below, the present invention does not produce a carbon fiber body having a specific pore or capillary volume by subsequent reinfiltration of a carbon body with additional resin.
Carbon-silicon carbide (C/SiC) brake preforms are conventionally made by melt infiltrating (MI) silicon into the pores of a carbonized carbon fiber/graphite preform. A process for doing so is depicted in FIG. 1. Alternatively, as described for instance in U.S. Pat. No. 5,962,135, a colloidal ceramic solution comprising silicon dioxide may be infiltrated into a carbon/carbon preform and therein converted to a refractory material of silicon carbide.
When polyacrylonitrile (PAN) fibers are used in a melt infiltration process, they are conventionally adhered into a preform with phenolic-based resins. However, this approach requires that the fibers be protected with a chemical vapor deposition (CVD) pyrolytic carbon layer, so that the fibers will not be damaged by the molten silicon during melt infiltration.
U.S. Pat. No. 6,129,868 is concerned with the manufacture of high density carbonxe2x80x94carbon preforms for friction materials. The patent discloses thermomechanical pressing method using any carbon fiber (including PAN) and industrial coal tar or petroleum pitches having a melting point of 176-662xc2x0 F. (80-350xc2x0 C.). Column 1, lines 42-54. In this context, it teaches that an xe2x80x9c. . . industrialized petroleum pitch exhibited better wetting ability compared to high-temperature coal-tar pitchesxe2x80x9d. Column 2, lines 23-31.
U.S. Pat. No. 5,753,018 relates to improved binder resin mixtures for vehicular brake linings. In the paragraph bridging columns 1-2, this patent teaches that xe2x80x9cAlternatives to phenolics have been made available for years, but their use has been limited to specialized applications. Some of the evaluated resin binders such as coal tar and petroleum pitches are quite brittle, despite having a low cost and a high char yield.xe2x80x9d
U.S. Pat. No. 3,932,568 relates to molded carbon base or graphite base articles for use in braking systems. This patent teaches, relative to the character of the binder used for coating and adhering together the carbon particles in such articles, that xe2x80x9cThe pitches and tars usually used in making graphite articles heretofore are unsuitable. Resins of the phenolic type must be used.xe2x80x9d Column 4, lines 31 and following.
In accordance with the present invention, carbon-silicon carbide brake preforms are manufactured by carbonizing a blend of carbon (e.g., polyacrylonitrile) fibers and thermosetting pitch resin, optionally along with a filler such as graphite, to provide an intermediate product having open porosity and subsequently filling the pores of the intermediate product with silicon by a melt infiltration process.
Molded articles that consist principally of carbon, that have relatively high strength and resistance to decomposition by frictional heat, and that are suitable for melt infiltration with silicon, are produced by, e.g.: coating randomly oriented polyacrylonitrile fibers, optionally mixed with finely divided carbon powder, with a thermosetting blend of (a) pitch and (b) an organic medium, at an elevated temperature to form a viscous molding compound; molding the compound at a low pressure and elevated temperature so that a solid compact is obtained; stripping a molded article made of said compound from the mold; optionally heating the molded article at gradually increasing temperatures to insure complete condensation within the article; and carbonizing the condensed molded article in an inert atmosphere at gradually increasing temperatures to produce a molded C/C intermediate article suitable for melt-injection with silicon. The molded preform intermediates may be configured in the form of a disc brake rotor or a disc brake pad.
It has been discovered that pitch-based thermosetting resin systems can form extremely strong bonds with the surfaces of carbon fibers such as PAN fibers, and that these bonds are maintained through the carbonization step of the preform manufacturing process. One example of these pitch-based thermosetting resin systems is a 50:50 weight percent mixture of coal tar pitch and 2-furaldehyde. These resin mixtures are catalyzed using an acidic species such as p-toluenesulfonic acid. Those skilled in the art will readily appreciate that other thermosetting mixtures of pitch and organic solvents (e.g., furfuryl alcohol) and other acidic catalysts, to initiate a thermosetting crosslinking reaction, will provide similar results.
As a result of the strong bond between the thermosetting pitch resins and the carbon fibers, the fiber bundles in the preform retain their integrity and are protected from attack by molten silicon during the melt infiltration process. This approach permits the omission of the costly CVD step required in conventional PAN preform manufacture or other intermediate processing operations such as reimpregnation with additional resins followed by resin curing and carbonization.
An added benefit of the pitch-based resin approach of this invention is that the resin mixture has a low viscosity and cures at temperatures of as low as 266xc2x0 F. (130xc2x0 C.). These low temperatures greatly extend the life of mold release coatings used in dies to manufacture molded parts. The low resin viscosity permits complex parts to be molded from mixtures of PAN fibers and pitch resin without resorting to the high pressures needed for conventional phenolic-based chopped fiber molding compounds. Also, this invention enables resin transfer molding of dry carbon fibers at low pressures and temperatures.